Stability and site occupancy of Fe4+in Ca-doped YIG films

The nature and site occupancy of Fe⁴⁺centers in Ca-doped YIG films is investigated by studying the symmetry properties of the near-infrared absorption. We apply the recently proposed AVIA (angular variation of induced anisotropy) method: we measure, as the single- -crystal film is rotated relative to a magnetic field, the angular variation of the magnetic linear dichroism, which is the anisotropy property utilized in this case. The AVIA pattern thus obtained is compared with the theoretical angular variations appropriate to the symmetry of the garnet sites. The result we find is a uniform distribution of Fe⁴⁺over the a (octahedral) and d (tetrahedral) sublattices. This uniform distribution does not change, in spite of varying Fe⁴⁺concentration, after successive annealing treatments in oxidizing or reducing atmospheres. The structural changes through the annealing steps are monitored by X-ray diffraction and the lattice is found to expand, under reducing treatments, only as long as there are Fe⁴⁺centers. After all Fe⁴⁺ions have been reduced (namely at zero near-infrared absorption), rapid diffusion of oxygen anions is hindered, in spite of the large concentration of oxygen vacancies. All observed effects are explained by a single mechanism: electron deficiency (or holes), in Ca-doped YIG, results in a widespread distribution of Fe4+and, effectively, in the presence of O- oxygens.